The field covers the combination of an opposed-piston engine with a hypocycloidal drive. In addition, the field covers the use of a piston coupled to a hypocycloidal drive to generate electrical power.
The opposed piston internal-combustion engine was invented by Hugo Junkers around the end of the nineteenth century. In Junkers' basic configuration, two pistons are disposed crown-to-crown in a common cylinder having inlet and exhaust ports near bottom dead center of each piston, with the pistons serving as the valves for the ports. The engine has two crankshafts, each disposed at a respective end of the cylinder. The crankshafts are linked by rods to respective pistons and are geared together to control phasing of the ports and to provide engine output. The advantages of Junkers' opposed piston engine over traditional two-cycle and four-cycle engines include superior scavenging, reduced parts count and increased reliability, high thermal efficiency and high power density.
Nevertheless, Junkers' basic design contains a number of deficiencies among which is excessive friction between the pistons and cylinder bore caused by side forces exerted on the pistons. Each piston is coupled by an associated connecting rod to one of the crankshafts. Each connecting rod is connected at one end to a piston by a wristpin internal to the piston; at the other end, the connecting rod engages a crankpin on a crankshaft. The connecting rod pivots on the wristpin in order to accommodate circular motion of the crank pin. As the connecting rod pushes the piston inwardly in the cylinder, it exerts a compressive force on the piston at an angle to the axis of the piston, which produces a radially-directed force (a side force) between the piston and cylinder bore. This side force increases piston/cylinder friction, raising the piston temperature and thereby limiting the brake mean effective pressure (BMEP) achievable by the engine.
An engine coupling invented by Mathew Murray in 1802 converted the linear motion of a steam engine piston and rod into rotary motion to drive a crankshaft by a “hypocycloidal” gear train coupling the rod to the crankshaft. A hypocycloid is a special plane curve generated by the trace of a fixed point on a small circle that rolls within a larger circle. In Murray's gear train, the larger circle is the “pitch circle” of a ring gear with teeth on an inner annulus and the small circle is the pitch circle of a spur gear with teeth on an outer annulus. (See the definition of “pitch circle” in American National Standard publication ANSI/AGMA 1012-G05 at 4.5.3.1.1, page 10). The spur gear is disposed within the ring gear, with its teeth meshed with the teeth of the ring gear. As the spur gear rotates, it travels an orbit on the inner annulus of the ring gear. Murray's gear train represents a special hypocycloid in which the pitch diameter (D) of the ring gear's pitch circle is twice the pitch diameter (d) of the spur gear's pitch circle. When D=2d, a point on the spur gear pitch circle moves in a straight line along a corresponding pitch diameter of the ring gear as the spur gear orbits within the ring gear. Murray connected one such point to a piston rod; the linear motion of the piston rod caused the spur gear to revolve within the ring gear, and the gear train converted the piston's linear motion to rotary motion.
Cycloidal gear arrangements have been used in numerous internal combustion engine configurations, including opposed piston engines. See U.S. Pat. No. 2,199,625, for example. In the engine disclosed in the '625 patent, opposed pistons are coupled to cycloid crank drives by means of connecting rods. However, the '625 patent omits two critical insights in this regard.
First, the plane curve traced by the spur gear is not linear in any embodiment taught in the '625 patent: thus, connecting rod motion is not linear. In fact, each connecting rod conventionally engages a wristpin internal to a piston, which allows the connecting rod to pivot with respect to the axis of the piston in order to accommodate the non-linear plane curves traced by the spur gear. Consequently, as the connecting rod pivots on a return stroke while moving a piston into a cylinder, it imposes side forces on the piston, which causes friction between the piston and cylinder bore.
Thus, an unrealized advantage of coupling the pistons of an opposed piston engine to hypocycloidal drives in which the ratio between the pitch diameters of the ring and spur gears is 2:1 is that the pistons, and their connecting rods, undergo purely linear movement along a common axis, thereby eliminating radially-directed side forces that cause friction between the pistons and the bore of the cylinder in which they are disposed.
The '625 patent does indicate that grafting a hypocycloidal output to an opposed piston engine construction can add a dimension of flexibility to engine design and operation. For example, the ratio between the pitch diameters is varied to accommodate piston strokes of varying length, which, according to the patent, can be tailored to improve scavenging and piston cooling. However, the '625 patent omits the case where D=2d, in which the linear motion of the spur gear is sinusoidal. The '625 patent therefore lacks a second critical insight: the sinusoidal characteristic of the resulting linear motion can support useful adaptations of a hypocycloidally-coupled engine to produce a desirable sinusoidal output. For example, an internal-combustion engine may be adapted to generate AC electrical power by mounting a coil to the skirt of a piston and coupling the piston to a hypocycloidal drive in which D=2d. The action of the hypocycloidal drive imposes a sinusoidal period on the straight linear motion of the piston. As the piston transports the coil though a magnetic field, a sinusoidal voltage is induced in the windings of the coil.